Ring laser angular rate sensors, sometimes referred to as ring laser gyros, are well known in the art. A detailed description may be found in the "Background of the Invention" of U.S. Pat. No. 4,597,667, which is incorporated herein by reference. FIG. 1 shows schematically a ring laser gyro system. Briefly, such sensors include a ring laser gyro 10 supported in a block 5 having a plurality of gas containing tunnels (not shown). At the intersection of the tunnels are mirrors 13, 15, and 18 that define a closed-loop optical path 16 which is traveled by counter-propagating laser beams therein. Practical embodiments of ring laser angular sensors usually include a path length control apparatus. The purpose of the path length control apparatus is to maintain a constant path length for the counter-propagating laser beams. Maintaining a constant path length avoids false rotation errors from the laser gyro. The path length control function is usually provided by an arrangement wherein at least one of the mirrors is attached to a piezoelectric transducer which controls translational movement of the mirror or mirrors. The transducer effects the laser beam path length in response to a drive signal provided through a transducer drive amplifier. In the example shown in FIG. 1 mirrors 13, 15 are path length control mirrors. Mirrors 13, 15 move along paths represented by arrows 12, 14 in response to path length control signals.
One technique for maintaining a constant path length is to detect the intensity of one or both of the laser beams and control the path length of the ring laser such that the intensity of one or both of the beams is at a maximum. U.S. Pat. No. 4,152,071 which issued May 1, 1979 to T.J. Podgorski, and is assigned to the assignee of the present invention illustrates a control mechanism and circuitry as just described. Path length transducers for controlling the path length of the ring laser are well known, and particularly described in U.S. Pat. No. 3,581,227, which issued May 25, 1971 to T.J. Podgorski, also assigned to the assignee of the present invention, U.S. Pat No. 4,383,763, which issued May 17, 1983 to Hutchings et al and U.S. Pat. No. 4,267,478, which issued May 12, 1981 to Bo H.G. Ljung, et al. All these patents are incorporated herein by reference.
In the aforementioned patents, the beam intensity is either detected directly as illustrated in the aforementioned patents, or may be derived from what is referred to as the double beam signal such as that illustrated in U.S. Pat No. 4,320,974, which issued on Mar. 23, 1982 to Bo H.G. Ljung, and is also incorporated herein by reference.
Herein "mode" is defined as the equivalent of one wavelength of the laser beam. For a helium-neon laser, one mode is equal to 0.6328 microns which is equal to 24.91 micro-inches.
In path length control systems of the prior art, the path length control finds mirror positioning for which the lasing polygon path length, i.e., the ring laser path length, is an integral number of wavelengths of the desired mode or frequency, as indicated by a spectral line, of the lasing gas. With proper design, the path length control forces the path length traversed by the laser beams to be a value which causes the laser beams to be at maximum power.
As is also known in the prior art, ring laser gyros are subject to small bias drift errors. Bias drift errors can result in significant inaccuracies if the ring laser gyros are operated for extremely long periods of time.
Now referring to FIG. 2 which shows the results of new, unpublished experiments conducted by Honeywell Inc. of Minneapolis, Minn. which imply the existence of a ring laser gyro bias drift that is periodic. The typical bias magnitude change 22 was on the order of (+/-) 0.01.degree./hr about a mean value shown as line 21 in FIG. 2. Bias magnitude changes, shown as curve 22, were observed to be sinusoidal in nature with respect to mirror position shown as the X axis 19 in FIG. 2. The plot in FIG. 2 shows the bias magnitude change curve 22 in relation to the single beam signal curve 24. The single beam signal curve 24 is derived from the magnitude of the AC component of the laser intensity monitor signal. Experimentally the bias was found to be 90.degree. out of phase, as shown by magnitude 26, with the single beam signal curve 24 (SBS) but equal in period. Typically the average bias crossings 25 and 27 of the BIAS sinusoid curve 22 occur at the minimum or maximum of the SBS signal curve 24.
The bias curve 22 is shown varying sinusoidally during one period of movement of the two mirrors 13 and 15. One period of movement is equivalent to two wavelengths. Even though the mirrors are moving the system maintains a constant laser path 16 in the laser gyro 10 as shown in FIG. 1.
The plot of FIG. 2 implies that as one mirror is moved "out" one wavelength and the other mirror is moved "in" one wavelength, for a total of two wavelength changes, the bias in the laser gyro 10 varies over one complete period. Ideally the bias will vary uniformly as the mirrors are moved from an average bias point 25 to a negative maximum bias point 26 through an average bias again at point 27 to a maximum bias at point 28 to return to an average bias at point 29. Those skilled in the art having the benefit of this new disclosure will recognize that with respect to the average bias 21 the integral of the bias curve 22 over one period of the curve from point 25 to point 29 is zero which implies that the total bias over the entire period is the average bias 21.
The present method and apparatus of the invention exploits the above described phenomena to accomplish a bias drift improvement.